(a) Field of the Invention
The present invention relates to an automatic transmission, and more particularly, to a damper clutch control method for an automatic transmission which reduces shock caused by specific operations of an accelerator pedal.
(b) Description of the Related Art
A damper clutch (D/C) is provided in a torque converter to enable engine torque to be directly used by the transmission without first being multiplied by the torque converter. That is, when vehicle driving conditions are such that engine torque does not require alteration before being supplied to the transmission, the D/C in the torque converter is engaged to enable the direct supply of engine torque to the transmission.
The scope of operation of the D/C is determined by rpm of a turbine, which is linked to the transmission, and by throttle valve opening. If the turbine rpm and throttle valve opening are at predetermined levels (i.e., predetermined combinations), a transmission control unit (TCU) outputs a drive signal to a D/C control solenoid valve to operate the D/C.
FIG. 1 shows a graph of operating and non-operating regions of the D/C. As shown in the drawing, throttle opening and turbine rpm Nt must be at specific levels for the D/C to operate. That is, certain combinations or ratios of throttle opening and turbine rpm define a region B during which the D/C is engaged, whereas at other regions A and C, which are defined by other specific combinations of throttle opening and turbine rpm, the D/C is not engaged.
During the operation of the vehicle, the driver may at times abruptly release his foot from the accelerator pedal such that the throttle valve is closed. When this occurs, lift-foot-up (LFU) shifting is performed. At this time, the driver may again depress the accelerator pedal, which is referred to tip-in. In either of these situations, throttle valve opening, turbine rpm Nt and engine rpm Ne vary as shown in FIG. 2 so that the D/C is disengaged.
FIG. 2 shows a graph of throttle valve opening and corresponding turbine rpm Nt, engine rpm Ne and torque T during LFU shifting and tip-in. As shown in the drawing, engine rpm Ne and turbine rpm Nt increase and decrease in accordance with changes in throttle valve opening. Reference numerals 3 and 4 in the drawing indicate points at which shock occurs. In more detail, when throttle opening is abruptly reduced, a corresponding drop occurs in torque T. Torque T continues at this low level until turbine rpm Nt equal engine rpm Ne (i.e., where shifting is completed). At this point, LFU shock 3 occurs. In a state where engine rpm Ne are less than turbine rpm Nt, if the accelerator pedal is again depressed by the driver so that throttle opening is increased, tip-in shock 4 occurs.
With the disengagement of the D/C, engine rpm Ne are reduced and vehicle speed increased. As a result, shock occurs and the overall ride comfort of the vehicle deteriorates.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a damper clutch control method for an automatic transmission which reduces shock generated during LFU shifting and tip-in by increasing an operating scope of a D/C.
To achieve the above object, the present invention provides a damper clutch control method comprising the steps of detecting a throttle opening, an engine rpm and an turbine rpm; performing lift-foot-up shifting if throttle opening is abruptly decreased; determining if a difference in engine rpm and turbine rpm is below a predetermined value; and duty controlling a damper clutch if the difference in engine rpm and turbine rpm is below the predetermined value.
According to a feature of the present invention, the predetermined value of the difference in engine rpm and turbine rpm is approximately 100 rpm.
According to another feature of the present invention, the step of duty controlling the damper clutch is performed for a predetermined duration and at a predetermined duty, after which the duty is controlled such that engine rpm and turbine rpm are equal.